[Qgis-user] results Measure Tool vs. length()

[Michel Stuyts]

I understand that every projection has a distortion, but I was wondering how the QGIS Measure Tool manages to give results very close to local projections anywhere in the world no matter what project CRS is set, but the length() function can't.

Michel


________________________________
Van: Micha Silver [tsvibar at gmail.com]
Verzonden: vrijdag 18 mei 2018 17:41
Aan: Michel Stuyts; qgis-user
Onderwerp: Re: [Qgis-user] results Measure Tool vs. length()

Maybe I misunderstood your question, but you *should* expect different length, area and direction calculations when converting from one projection to another. Every projection distorts. Either distance, area, or direction, or some combination. A local projection is designed to minimize distortion for some small section of the planet. So survey measurements in that region will closely match map calculations. But trying to calculate from a layer that's projected to some other projection will always give different results.

(If you already understand the above, I apologize for restating the obvious...)

On 05/18/2018 01:14 PM, Michel Stuyts wrote:
Thank you very much for the explanation.

When I use length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:31370')) in Belgium the results are very similar to the result of the Measure Tool, but if use this expression in New York the result is different from the result of the Measure Tool.  It is the other way around if I use length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:102316')). What kind of projection does the Measure Tool use to calculate a length? Because the results of the Measure Tool seem to be very similar to local projections.


Michel





Van: Julien Guilloux [mailto:julien.guilloux at ecrins-parcnational.fr]
Verzonden: vrijdag 18 mei 2018 11:01
Aan: Michel Stuyts <michel.stuyts at edegem.be><mailto:michel.stuyts at edegem.be>; Springfield Harrison <stellargps at gmail.com><mailto:stellargps at gmail.com>; qgis-user <qgis-user at lists.osgeo.org><mailto:qgis-user at lists.osgeo.org>
Onderwerp: Re: [Qgis-user] results Measure Tool vs. length()

Hi Michel,

Never use epsg 3857 for measurement beacuse it is not a "projection"...
this "pseudo projection mercator" 3857, called often "Popular Visualisation"  is pseudo.... Because only use for visualization applications.

example in France :  3200 km for a road for example, instead of 2100 km in a real projection (as local zone UTM or a Lambert projection)


So, you can use qgis, esri, postgis etc... the measurment with 3857 will be always bad, due to a "false" calcul from origine 0,0 (equateur cross greenwich) and "false" ellipoide etc....

best regards,
Julien

have a look on :
http://spatialreference.org/ref/sr-org/6864/

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Julien-Pierre GUILLOUX
Chargé de mission Eau, Forêt, Natura 2000, coordinateur des porter-à-connaissance

Parc national des Écrins
04 92 40 20 56 / 06 99 77 38 26
www.ecrins-parcnational.fr<http://www.ecrins-parcnational.fr/>





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--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
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